Introduction: In the 1930s, YEDLs predecessor, the Yorkshire Electric Power Company, built a range of long-spanned 11, 33 & 66 kV steel girder mast lines, commonly known as the Woodhouse mast designs.

The Woodhouse steel mast supports were originally designed to accommodate 0.15” (7/.166”) HDBC & 0.1” (7/.136”) HDBC conductors using three basic design spans of 500 ft, 650 ft and 700 ft. A number of the circuits were later reconductored with 0.175” (37/.110”) conductors designed on a 700 ft basic span. This was achieved without the need to carry out any alterations to the existing supports.

Over time the majority of 11 kV supports have been replaced with traditional wood pole alternatives, the majority of them resulting in interpoling as a means to reduce the long span lengths. A similar approach has been applied to the 33 and 66 kV circuits, but difficult wayleaves situations mean that around 300 km still remain.

The original specification cannot be re-used as it does not conform to current overhead line design requirements. Existing current designs such as AP1 or our proprietary OHL 9 and CE/C/37 specifications are limited to a span length of around 150m.

Objectives: The challenge facing Northern Powergrid is that they have around 300 km of an asset that is critical to system security and, dependent on land use, a hazard to the public as it deteriorates. The absence of a like-for-like replacement renders it difficult to obtain wayleaves for routes using current designs. This has already led to our undergrounding one circuit in its entirety (Thurcroft-Mexborough-Edlington) and a high-risk section of another (the tee to Crowle). The high costs involved were justified by the pressing need to replace these lines before they posed an unacceptable risk to public safety.